The manufacture of paper typically involves the processing of a carefully prepared aqueous fiber suspension to produce a highly uniform dry paper sheet. Three steps included in the typical process are sheet forming, where the suspension is directed over a porous mesh or "wire" upon which fibers are deposited while liquid filters through the wire; sheet pressing, where the formed sheet is passed through presses covered with porous "felt" to extract retained water from the sheet, to improve the sheet's uniformity, and to impart surface quality to sheet; and paper drying, where residual water is evaporated from the sheet. The sheet may then be further processed into the finished paper product.
It is well known that evaporation of water is energy intensive and thus relatively expensive. Consequently, efficient papermaking is dependent upon extracting water during the forming and pressing operations, and avoiding sheet defects which render the dried sheet unfit for use. Felts and wires are thus particularly important because they affect not only water removal but, because of their intimate contact with the sheet, the quality of the sheet itself. Deposits allowed to collect on the felt or wire can affect its water removal efficiency, can cause holes in the sheet, and can be transferred to the sheet material to create defects.
The quality of the aqueous fiber suspension used to produce the sheet is dependent upon many factors, including the wood and water used as raw materials, the composition of any recycled material added to the process, and the additives used during preparation of the suspension. Thus a variety of dissolved or suspended materials can be introduced into the manufacturing process, including both inorganic materials such as salts and clays, and materials which are organic in nature such as resins or "pitch" from the wood, as well as inks, latex, and adhesives from recycled paper products. A build up of deposits containing inorganic and/or organic materials on felts and other sheet forming equipment during the manufacturing process is recognized as a troublesome obstacle to efficient papermaking. Particularly troublesome are the sticky materials such as glues, resins, gums and the like which are associated with recycled fibers.
Methods of quickly and effectively removing deposits from the papermill sheet forming equipment are of great importance to the industry. The paper machines could be shut down for cleaning, but ceasing operation for cleaning is undesireable because of the consequential loss of productivity. On-line cleaning is thus greatly preferred where it can be effectively practiced.
The wire belt or cylinder used for sheet forming cycles continuously, as a belt, during production. The sheet-contact portion of the cycle begins where application of the fiber suspension to the wire belt or cylinder is started and continues until the formed sheet is separated from the wire surface; and the return portion of the cycle returns the wire from the position where the formed sheet has been removed from its surface to the beginning of the sheet-contact portion. With wire belts such as Fourdrinier wires, on-line wire cleaning has generally been performed during the return stage (i.e. where the wire is not in contact with the forming sheet) by treating the returning wire with a cleaning liquid (typically water); often by showering the wire with liquid under pressure. The showers can be assisted by mechanical surface cleaning. Use of water showers, with or without mechanical assistance, has not proved entirely satisfactory in preventing a build-up of either organic compounds or inorganic deposits on the wires, and additional materials have been used to provide cleaning liquids which are more effective. Predominantly fibrous or inorganic materials have been successfully removed using water-based formulations containing either acids or alkalis formulated with other chemicals such as surfactants. Where organic deposits are prevalent, they have been removed with some success by using organic solvents, including some formulations containing aromatic compounds with low flash points or chlorinated hydrocarbons. In some machines fine-pored fabric belts are now used instead of the more traditional wires.
Papermill felts also commonly circulate continuously in belt-like fashion between a sheet contact stage and a return stage. During the sheet contact stage water is drawn from the sheet usually with the aid of presses and/or vacuum into the pores of the felt. A clean felt, having fine pores which are relatively open, is especially desirable for effective paper manufacture since this allows efficient removal of water from the paper sheet. A felt cleaning procedure should remove both organic and inorganic deposits of both a general and localized nature, maintain felt porosity, and condition the fabric nap without chemical or physical attack on the substrate. Mechanical removal, typically by blade contact, has been used to remove debris from the felt surface. However, cleaning liquids are also utilized to remove troublesome build-up of organic and inorganic deposits. The fabric composition and conformation of many papermill felts makes them susceptible to chemical degradation. The cleaning chemicals should be easily removed by rinsing. Both continuous and shock cleaning is used in most papermills. The chemicals used include organic solvents, often chlorinated hydrocarbons. Acid and alkali based systems are also used, but at lower concentrations than used in wire cleaning. High concentrations of alkali metal hydroxides are often unsuitable for felt cleaning as they "attack" the fabric material.
Some of the more successful organic solvents have been identified as health risks, such as carcinogens, and thus require especially careful handling. Other solvent based products can damage plastic or rubber components used in the paper forming process. One on-line treatment of felts which we know has been used for several years with some success involves contacting the felt with aqueous solution of cationic surfactants such as alkyldimethyl benzyl ammonium chloride wherein the alkyl group consists of a mixture of C.sub.12 H.sub.25, C.sub.14 H.sub.29 and C.sub.16 H.sub.33 groups. However, experience has shown that some sticky materials still tend to adhere to felts despite treatment with these surfactants. Another felt conditioning practice which has been advocated in the past is application of aqueous solutions of cationic polymers to the felts. However this type of treatment can actually lead to a build-up of deposit of materials derived from the cationic polymers themselves.
Other sheet forming equipment such as deckers, filters, screens, and rolls can also become fouled. The process problems and treatments are, as a general rule, similar to the felt system, although certain considerations such as maintaining porosity and avoiding chemical degradation of fabric, which are important in felt cleaning and cleaning certain other fine-pored equipment components, may not be so critical for this other equipment.
Natural resin or gum in fresh wood can vary, depending on the species. Some types of pine wood, especially those containing 2 weight percent or more of resin, are commonly used in only very low percentages due to the gum and resin problems they cause. Papermakers alum or sodium aluminate have been traditionally used to control natural wood resin deposits. These products are added into the total pulp system with the objective of depositing the resin on the fiber. The effectiveness of this approach is limited by such factors as pH, the potential for corrosion, paper sheet formation, and the need to control interaction with other chemicals in the pulp system. Treatments which would permit the unrestricted use of these problem pine wood sources could have significant beneficial economic impact on some pulp and paper producers.
The increasingly more common use of recycled fiber has contributed to more serious build-ups of sticky material during paper formation. The glues, resins, gums, etc. which are found in recycled, secondary fiber tend to adhere to various parts of the paper-forming machine and to resist on-line shower cleaning. The materials which adhere to the felt can seriously affect drainage and paper formation. The end result in the product is holes, and ultimately, in some cases, breaks in the sheet during paper processing. Frequent shutdown may be necessary to solvent wash the felt to remove the particularly sticky material associated with recycled fiber. The advantages of paper recycling can thus be somewhat offset by reduced productivity of the papermaking machines.
Certain organic cleaners which were used frequently in the past have become environmentally undesirable. Thus, greater need has developed for cleaners which remove organic deposits without presenting an environmental hazard. Naturally, formulations used should not be destructive of the felts or other sheet forming equipment. While some materials have been considered to perform satisfactorily under certain conditions, there is still a continuing need for more effective deposit control agents for paper forming, particularly where recycled fiber is used as a raw material.
Another approach to deposit control has been the use of pulp additives such as anionic aryl sulfonic acid-formaldehyde condensates or cationic dicyandiamide-formaldehyde condensates. The additives may function for example as sequestrants, dispersing agents or surface active agents. In particular the cationic dicyandiamide-formaldehyde aminoplast resins have been described as bringing about the attachment of pitch (e.g. resinous matter and gums), in the form of discrete particles, to pulp fibers so that the pitch particles are uniformly distributed on the fibers themselves. Consequently, the amount of pitch which accumulates on the papermaking machine is reportedly reduced without causing dark spots or specks of pitch in the paper product.